This invention relates generally to a method and apparatus for providing efficient and sensitive x-ray imaging equipment. More particularly, the invention relates to a method and apparatus for cooling charge-coupled device-containing imaging systems and the like.
Two-dimensional charge-coupled device (CCD) arrays can be substituted for tube-type cameras or film as the imaging device in imaging systems. In the most straightforward operational mode a CCD image is acquired as a snapshot, analogous to the operation of a photographic camera. Due to their high sensitivity, CCD imaging systems are ideally suited for digital imaging of x-ray images as well as other types of images. For example, CCD imaging systems are used routinely in astronomy and have been used recently in the field of molecular genetics for digital imaging of electrophoretic gels and hybridization blots. Achim E. Karger et al., Line Scanning System for Direct Digital Chemiluminescence Imaging of DNA Sequencing Blots, 65 Anal. Chem. 1785 (1993).
An area CCD array is composed of two major functional components, a parallel register consisting of a two-dimensional array of light-sensitive CCD elements, organized, for example, in 576 lines of 384 elements each for a 586.times.384 pixel array, and a single-line linear output register positioned along one side of the parallel register and connected to the on-chip amplifier. Photons striking the CCD surface generate photoelectrons that are trapped in the CCD element nearest the location of photon incidence. After the array has been exposed to light, readout of the photogenerated charge is performed by simultaneously shifting the charge packets of all lines in parallel toward the output register. Having arrived at the output register, the charge packets are shifted one by one in the perpendicular direction toward the on-chip amplifier for subsequent analog to digital conversion.
Cryogenically cooled, low-noise CCD imaging systems are particularly well suited for low light level imaging because of their high sensitivity, linear response to incident photons, and wide dynamic range. CCD imaging systems maintained at low temperature are highly sensitive because of improved signal-to-noise ratios over systems maintained at higher temperature. Low temperature operation of CCD imaging systems increases the range of light levels readable by the imaging system, thus providing a wide dynamic range of incident light levels that can be imaged. Further, low temperature operation of CCD imaging systems provides for realization of the full utility of high resolution chips used with the imaging system. CCD systems used in the environment are typically categorized as being in the "scientific class," and generally acquire data slowly.
Faster scan CCD systems, used in conventional 30 frame per second acquisition, can also benefit from active cooling. A compact, yet effective, design for active cooling would be beneficial since it is desirable to take advantage of the inherent compactness that a CCD imaging system allows. Thus, a compact way of removing heat would be important, instead of depending on more bulky conventional methods, like fans and close proximity heat sinks.
In view of the foregoing, it will be appreciated that providing a method and apparatus for cooling CCD imaging systems and the like that avoid the problems and disadvantages of conventional methods and apparatus would be a significant advancement in the art.